Well, a great plot might be that the OC is an artist who can draw the secrets of the Poneglyphs without actually reading them. Robin is fascinated by this ability and they work together to uncover the true history of the world. Also, perhaps the OC has a connection to Robin's past, like knowing someone who was important to her family, which creates an emotional bond between them.
An interesting plot could involve the OC being a time - traveler who has come from a future where the world's history has been fully revealed. They meet Robin and start to show her glimpses of what they know. This causes a stir among the Straw Hats as they try to figure out if this information is reliable. The OC and Robin then have to prove the authenticity of this knowledge while also facing threats from those who don't want the truth to be known.
One idea could be that during a dangerous adventure, Zoro and Robin get separated from the crew. They have to rely on each other's skills. Zoro's combat abilities and Robin's knowledge of ancient texts help them navigate through an ancient, forgotten island filled with mysteries and traps.
A great plot idea could be that the OC is a healer who saves Ace's life after a particularly brutal fight. Ace is initially distrustful but as he recovers, he sees the kindness in the OC. They then go on a journey to find a rare herb that can cure a disease that is spreading among the pirates, and along the way, their relationship blossoms.
One interesting plot idea could be that Aokiji, after leaving the Marines, crosses paths with Robin. They start to share their knowledge of the lost history. Aokiji's past experiences in the Marines and his knowledge of certain secrets could complement Robin's search for the true history. Another plot could involve them teaming up against a common enemy who is trying to destroy all evidence of the Void Century. They use their unique abilities, Aokiji's ice powers and Robin's ability to sprout body parts on any surface, to fight off the enemy and protect the ancient artifacts related to the One Piece world's history.
One idea could be that the OC is a long - lost relative of a Straw Hat member. For example, an OC who is related to Luffy and has a unique power of their own, like the ability to control the weather in a small area. This could lead to interesting storylines where the OC helps the crew during difficult battles or in traversing dangerous seas.
Maybe the OC male is a chef on a ship and his amazing cooking skills attract a harem of female pirates. They are drawn to his ability to create delicious meals from the scarcest of ingredients in the middle of the ocean. His kind nature in sharing food with everyone also makes him endearing to them.
The OC could be a pirate from a rival crew. They meet Kid during a battle. Instead of fighting to the death, they end up in a situation where they have to team up against a greater enemy. During this time, they get to know each other better, and a relationship between Kid and the OC starts to form.
Well, perhaps the OC is from a long - lost tribe that has a connection to a hidden treasure. Corazon, being in the Donquixote Pirates, stumbles upon the OC. At first, there's a lot of mistrust, but as they face various challenges together, like dealing with other pirates who also want the treasure or escaping from the Marines, they start to trust each other more and more. And the OC could also have some knowledge about the Will of D that intrigues Corazon.
Well, an interesting plot could be an OC who was originally a civilian but got forced into CP9 due to some family secrets. This OC could then struggle between following the harsh orders of CP9 and their own moral compass. They might also form an unlikely alliance with some of the Straw Hat Pirates while on a mission, which would put them in a difficult position within CP9.
One idea could be that the Killer OC has a unique devil fruit ability that allows them to control time in a limited area. They could use this to outwit the Straw Hats during an encounter, creating a really intense battle scene.
One idea could be that the male reader is a new member of the Straw Hat crew. He and Robin gradually develop a bond during their adventures. For example, they could be exploring an ancient island together and the male reader saves Robin from a hidden trap. This shared danger could be the start of their relationship. Another plot could involve the male reader having a special power or knowledge that Robin is interested in, and through their interactions, they fall in love.